Production of alkyl nitrates



PRODUCTIUN F ALKYL NITRATEES Roy Bentley, Largs, Scotland, and MervynNisbet Thruston, Almondbury, Huddersfield, England, assignors toImperial Chemical industries Limited, a corporation of Great Britain N0Drawing. Filed June 16, 1954, Ser. No. 437,278

Claims priority, application Great Britain July 10, 1953 '6 Claims. (Cl.260-467) The present invention is concerned with a method of purifyingvolatile liquid nitric esters of monohydric alcohols and is particularlyconcerned with a method of purifying volatile liquid nitric esters ofmonohydric aliphatic alcohols obtained by distillation as from theproducts of an esterification reaction between the correspondingaliphatic alcohol and nitric acid, especially when the esterification isconducted with aqueous nitric acid in the presence of urea.

In the preparation of liquid nitric esters of the lower aliphaticalcohols by distillation it is usual to wash the crude liquid nitricester-containing portion of the condensate from the distillation withwater and subsequently with a mildly alkaline solution in order toremove any nitric acid and monohydric aliphatic alcohol present in it.

For certain of the aforesaid liquid nitric esters prepared in this wayit is found that even after the water and mildly alkaline washings theliquid nitric ester, although not affecting stainless steel, rapidly andprogressively corrodes ordinary ferrous metals such as mild steel, castiron and galvanised iron, with the production of a rusty deposit thatreadily flakes away from the metal. This corrosive action proceeds evenwhen air is excluded and when the ferrous metal is totally immersed inthe liquid nitric ester. Such corrosive samples of the aforesaid liquidnitric esters also attack copper and its alloys with the formation of ablue coating that adheres to the metal surface and they also attackaluminum slightly. It will be readily understood that these corrosiveproperties cause much inconvenience in the storage and transport of theliquid nitric ester and also in connection with its industrialutilisation, for example as a fuel burning alone or in presence ofoxygen, or as an additive to an oil fuel, since a liquid nitric esterexhibiting these properties cannot be stored or handled in conventionalferrous metal containers and cannot be passed through narrow passages,e.g. for spraying, especially in ordinary ferrous metal or copper alloystructures. These corrosive properties have been found to be shownparticularly by the volatile liquid nitric esters of monohydric alcoholshaving at least one methyl group close to the nitrate group as forinstance ethyl nitrate, isopropyl nitrate, and secondary butyl nitrate.There is furthermore evidence that variations in the conditions of anesterification have an efiect on the corrosive properties of theresulting volatile liquid nitric ester obtained by distillation from theproducts of the esterification reaction.

This corrosive effect is very easily shown on a piece of mild steelwholly immersed in the liquid nitric esters in a glass container as thepiece of mild steel develops a visible rust coating within 24 hourswhich increases in amount with increasing time of exposure leading tothe deposition of rusty particles at the bottom of the container. Theeffect has been observed most strongly with isopropyl nitrate and inlesser but still objectionable degree with ethyl nitrate, with secondarybutyl nitrate, and with ethoxy-ethyl nitrate prepared by nitration ofthe corresponding alcohols with aqueous nitric acid in the presence ofurea. Thus in the direct nitration method for the production ofisopropyl nitrate from isopropyl alcohol, aqueous nitric acid and urea,there is distilled and condensed from the reaction mixture a mixturecontaining isopropyl nitrate, aqueous nitric acid and isopropyl alcohol,which separates out into a layer rich in isopropyl nitrate and a layerrich in water. The isopropyl nitraterich layer is separated and washedwith water, which has the effect of removing isopropyl alcohol from itand diminishing its acidity. Even after water-washing, however, theisopropyl nitrate quickly develops acidity and it is found that mildsteel drums and galvanised iron drums are severely attacked by theproduct and aluminium containers are also somewhat corroded by it, whilecopper pipes acquire a blue bloom when it is passed through them.

it might be supposed that the residual acidity and corrosive propertiesof the product would easily be removed by washing the separatedisopropyl nitrate with a dilute solution of a mild alkali such as sodiumcarbonate, but this does not occur for on separation from the alkalinesolution and on standing the isopropyl nitrate is found to be stillharmful to the aforementioned metal vessels and its acidity may evenexceed that left after simple waterwashing.

These corrosive properties of the aforesaid volatile liquid nitricesters and the associated development of acidity after washing are notinherent properties of the volatile liquid nitric ester that exhibitsthem. We have discovered that the corrosive properties and the aciditythat develops in the water or dilute alkali washed volatile liquidnitric ester are due to the presence therein of an impurity consistingof a more readily hydrolysable ester than said nitric ester and formedas a result of the particular method of preparation of the nitric ester.The impurity is found to be the nitrous ester of the monohydricaliphatic alcohol and to be formed in the main in the gaseous phaseduring the distillation stage of the products of the esterificationreaction between the corresponding alcohol and nitric acid.

When a volatile liquid nitric ester of a monohydric aliphatic alcoholcontaminated with the corresponding nitrous ester as an impurity iswashed so that substantially all free nitric acid associated with it isremoved, e.g. by washing it with water and then with dilute alkali, andthe ester is separated from the aqueous washing medium, bydrolysis ofthe nitrous ester continues slowly in the presence of the water presentin the dissolved state in the product, and the acid products ofhydrolysis so generated give rise to the attack on the metals.

It has now been. found that a volatile liquid nitric ester of amonohydric aliphatic alcohol exhibiting these corrosive properties whenwashed in the aforesaid manner, and containing the corresponding nitrousester as an impurity, can be rendered substantially free from saidnitrous esters and can thus be prevented from developing corrosiveproperties, if the crude nitric ester is first washed with an excess ofan aqueous solution containing urea and a mineral acid in aconcentration to cause evolution of gas in the presence of the urea onlyas long as any nitrous ester is present and if the so treated nitricester is substantially freed from the acid used and excess of urea, asby treatment with water and a metallic carbonate or hydroxide or thelike neutralising agent.

According to the present invention the method of purifying volatileliquid nitric esters of monohydric aliphatic alcohols substantially fromnitrous ester comprises washing the nitric ester With an excess of anaqueous solution containing urea and a mineral acid in a concentrationto cause evolution of gas in the presence of the urea only as long asany nitrous ester is present and thereafter freeing the nitric estersubstantially from said acid and excess urea.

To be sure that sui'hcient urea is employed the percentage of thenitrous ester present in the volatile liquid nitric ester of themonohydric aliphatic alcohol may if desired be determined analyticallyas by a suitable titration method. In the absence of the mineral acidthe rate of evolution of gas is quite imperceptible and it may besupposed that the function of the mineral acid is to acceleratehydrolysis of the nitrous ester into the monohydric alcohol and nitrousacid, the nitrous acid then reacting immediately in known manner withthe urea in the molar ratio 2 l to produce nitrogen, water and carbondioxide, the destruction of the nitrous acid causing the hydrolysis toproceed progressively until the nitrous ester is used up and the gasevolution consequently ceases. The mineral acid concentration musttherefore be sufficient to cause the evolution of the gas to take placeat a visible rate so that the treatment can be completed within areasonable time. On the other hand, the mineral-acid concentration mustnot be suflicient to bring about any substantial hydrolysis of thevolatile liquid nitric ester, for this would not only result in the lossof the desired material but would also result in the consumption of ureain greater amount than that required for the nitrous acid resulting fromhydrolysis of the nitrous ester, so that the gas evolution would neverstop, and would ultimately result in the consumption of the whole of theurea so that the reaction would become violent.

Nitric acid may conveniently be employed as the mineral acid and it ispreferred that the acid concentrations of the aqueous solution incontact with the alkyl nitrate should be between approximately 1 and3'Normal, since these concentrations suitably accelerate the hydrolysisof the nitrous ester without resulting in any appreciable hydrolysis ofthe nitric ester. It is possible to employ the urea and the mineral acidin the form of the urea salt of the acid, since urea salts dissociatesatisfactorily in water, and it is especially desirable to use ureanitrate since this compound crystallises out on cooling from the wastenitric acid'formed in processes for the manufacture of the volatileliquid nitric esters of aliphatic alcohols when these alcohols aretreated with nitric acid in the presence of urea, for example incontinuous processes where a portion of the liquid reaction mixture fromwhich said nitric ester is distilled is run to waste. It is convenientto use a nearly saturated solution of urea nitrate in water. It is thusan advantage of the invention that it can be carried out with a materialavailable as a by-product in the production of the nitric ester to bepurified.

The treatment may advantageously be conducted by washing the impurevolatile liquid nitric ester with the solution containing the urea andthe mineral acid under such conditions as to enlarge the surface ofcontact between the two immiscible liquids, for example with mechanicalagitation or agitation by means of an air stirrer, the action of whichis discontinued when it is desired to observe the progress of theevolution of gas. The washing may advantageously be conducted at atemperature somewhat above atmospheric, e.g. at about 40 0., since thisaccelerates the destruction of the nitrous ester without materiallyafieoting the nitric ester, but the reaction also occurs satisfactorilyat room temperature. It is usually desirable that the crude volatileliquid nitric ester of a lower monohydric aliphatic alcohol should besubjected to a water washing before the treatment is commenced.

Whatever may be the mechanism of the reaction between the impurevolatile liquid nitric ester of the monohydric aliphatic alcohol and theaqueous solution containing the urea and the mineral acid, the fact isthat nitrogen and carbon dioxide are evolved and the nitric ester soonattains a condition in which after water washing and separation from thewash water its acidity no longer increases on storage and it can bestored for example in mild steel drums without damaging them.

The invention is illustrated in the following examples.

' temperature.

4 Example 1 Isopropanol is nitrated in a continuous manner byintroducing continuously separate streams of 75% isopropanol containing10% added urea and of 70% nitric acid into a nitration reaction mixtureof 40% aqueous nitric acid containing urea in a nitrator in which thelevel of the liquid is kept constant by a constant level overflow and bydistilling and condensing the volatile products so formed. Thecondensate consists of two layers. The lower layer is impure isopropylnitrate and is washed with two volumes of water to remove unreactedisopropanol. The amount of isopropyl nitrite in the water washedisopropyl nitrate is obtained by determining the specific gravity of thedried water washed product and by making use or a graph which recordsthe specific gravities for pure isopropyl nitrate, pure isopropylnitrite and mixtures of isopropyl nitrate and isopropyl nitrite of knowncomposition.

After the isopropyl nitrite content is determined the water washedproduct is thoroughly shaken for 20 minutes with its own volume of asolution of urea nitrate in water of such concentration that it issaturated at room This solution contains a considerable excess of ureanitrate over that amount required to destroy completely the isopropylnitrite present in the impure isopropyl nitrate. This solution is alsoeasy to prepare and is convenient to handle. The solubility of ureanitrate in water at room temperature is approximately 12% by weight. Theresulting mixture is allowed to separate into two layers whereof theupper layer consisting of isopropyl nitrate is separated 0E and washedwith twice its own volume of water in which it sinks.

The resulting lower layer of isopropyl nitrate is carefully separatedfrom the supernatant wash water. Its aciditycalculated as nitric acid is0.005% and it is found that this acidity does not increase on standing.The product can be stored in a mild steel drum for more than 6 monthsWithout effecting any noticeable corrosion. In a comparative test inwhich the isopropyl nitrate rich layer of the condensate is washeddirectly with water instead of with urea nitrate, it is found impossibleto reduce the acidity of the isopropyl nitrate layer below about 0.15%.After a weeks storage in a mild steel container the resulting isopropylnitrate causes relatively severe corrosion of the container. If theisopropyl nitrate layer of the crude condensate is washed with'dilutesodium carbonate solution instead of water its acidity after a shortstorage is not less than that which is obtained as a result of waterwashing and the product which is separated from the dilutealkalineliquor similarly corrodes a mild steel drum.

Example 2 Impure isopropyl nitrate, prepared as in Example 1, andexhibiting visibly corrosive properties on mild steel test piecesimmersed in it within one hour even in the dark is washed with an equalvolume of an aqueous solution of urea and nitric acid in which thenitric acid conccntration is approximately normal and the molar ratio ofurea to nitric acid is approximately 2:1. The washing is conducted at 40C. with mechanical agitation until the evolution of colourless gas whichbegins at a rate sufficient to cause the mixture to froth, is no longervisible. This takes about twenty minutes. The stirring is stoppedand'the ester layer, the denser one, is separated from the aqueous layerwhich is found still to contain urea in solution, and is washed withtwice its own volume of water. After the ester is separated from thewash water it is filtered through a column of marble chips. Test piecesof mild steel when immersed in the resulting product in the dark undergono visible tarnishing or corrosion over two weeks time.

Example 3 The ethyl nitrate-rich layer which is separated in thecondensed distallate' in the production of ethyl nitrate with aqueousnitric acid in presence of urea is treated with its own volume of asaturated solution of urea nitrate and is separated and washed in thesame manner as as the isopropyl nitrate-rich layer in Example 1. Theresulting product is of negligible acidity and can be stored for 6months in a mild steel drum without exerting disadvantageous corrosion,whereas a sample of the same ethyl nitrate-rich layer thoroughly washedwith water instead of urea nitrate solution has a noticeable acidity andcorrodes a mild steel container badly within a week.

Example 4 Secondary butyl nitrate prepared by distillation from aqueousnitric acid to which are added continuously separate streams ofsecondary butyl alcohol and nitric acid and exhibiting visibly corrosiveproperties on mild steel W test-pieces immersed in it within twelvehours, is washed with an equal volume of an aqueous solution of ureanitrate of such concentration that it is approximately saturated at roomtemperature. The resulting mixture is allowed to separate into twolayers, whereof the upper layer consisting of secondary butyl nitrate isseparated off and washed with an equal volume of a solution of 2% sodiumcarbonate. Test-pieces of mild steel immersed in the resulting productafter separation, undergo no visible tarnishing or corrosion over oneweeks time.

What we claim is:

1. A method of purifying volatile liquid nitric esters of monohydricalkyl alcohols substantially from nitrous ester which comprises washingthe nitric ester with an excess of an aqueous solution containing ureaand nitric acid in a concentration suflicient to cause visible evolutionof gas in the presence of the urea only as long as the nitrous ester ispresent but not sufiicient to cause any substantial hydrolysis of theliquid nitric esters, and thereafter freeing the nitric estersubstantially from said acid and excess urea.

2. A method as claimed in claim 1 wherein the acid concentration of theaqueous solution in contact with the alkyl nitrate is betweenapproximately 1 and 3 Normal.

3. A method as claimed in claim 1 wherein the urea and the nitric acidare in the form of the urea salt of the acid.

4. A method as claimed in claim 1 wherein the washing of the nitricester with an excess of an aqueous solution containing urea and nitricacid is carried out at about C.

5. A method as claimed in claim 1 wherein the nitric ester is subjectedto water washing before washing it with an excess of an aqueous solutioncontaining urea and nitric'acid.

6. In a method for preparing a purified noncorrosive volatile liquidnitric ester of a monohydric alkyl alcohol by reacting said alcohol withnitric acid and distilling off the nitric ester thus formed, theimprovement whereby nitrous ester formed in distilling olf said nitricester and admixed therewith to create a corrosive effect is removed,said improvement comprising the steps of washing the distilled nitricester containing nitrous ester with a saturated aqueous solution of ureanitrate until said nitrous ester is substantially completely removedwithout substantial hydrolysis of said nitric ester and thereafterseparating said nitric ester from said urea nitrate.

References Cited in the file of this patent UNITED STATES PATENTS1,852,041 Crater et a1. Apr. 5, 1932 1,967,551 Crawford July 24, 19342,166,698 Allen July 18, 1939 2,294,849 Olin et a1. Sept. 1, 1942

1. A METHOD OF PURIFYING VOLATILE LIQUID NITRIC ESTERS OF MONOHYDRICALKYL ALCOHOLS SUBSTANTIALLY FROM NITROUS ESTER WHICH COMPRISES WASHINGTHE NITRIC ESTER WITH AN EXCESS OF AN AQUEOUS SOLUTION CONTAINING UREAAND NITRIC ACID IN A CONCENTRATION SUFFICIENT TO CAUSE VISIBLE EVOLUTIONOF GAS IN THE PRESENCE OF THE UREA ONLY AS LONG AS THE NITROUS ESTER ISPRESENT BUT NOT SUFFICIENT TO CAUSE ANY SUBSTANTIAL HYDROLYSIS OF THELIQUID NITRIC ESTERS, AND THEREAFTER FREEING THE NITRIC ESTERSUBSTANTIALLY FROM SAID ACID AND EXCESS UREA.